Many commercial polyesters are copolymers containing polymerized terephthalic acid units and polymerized other dicarboxylic acid units, such as isophthalic acid units, along with polymerized dihydric alcohol units such as ethylene glycol units. The other dicarboxylic acid units modify the polymer properties in desirable ways, for example, by improving the clarity of molded articles, and by lowering the melting point of fibers made from such copolymers, thus making such fibers useful as binder fibers in polyester webs.
For example, commercial processes for the manufacture of such copolymers utilize mixtures of terephthalic acid and isophthalic acid, or mixtures of dimethylterephthalate and dimethylisophthalate. Dimethylisophthalate is commercially available, but is considerably more expensive than isophthalic acid. In commercial facilities that utilize dimethylterephthalate it is economically desirable to employ isophthalic acid as the source of the isophthalic units; however, as pointed out in British Patent 1,073,640 to Goodyear, "If isophthalic acid is substituted for the dimethylisophthalate . . . the ester interchange reaction between dimethylterephthalate and glycol is inhibited. . . " It is believed that the isophthalic acid causes the ester interchange catalyst, often a manganese catalyst, to become inactive and perhaps precipitate. This British patent found a solution to this problem by first reacting isophthalic acid and the glycol, then carrying out the ester interchange reaction of dimethylterephthalate and glycol in the presence of the bis-glycol isophthalate, and finally condensing the mixture to form the polyester copolymer.
It has now been discovered that the desired copolymers can be produced in a continuous process by first ester interchanging dimethylterephthalate and ethylene glycol to form a stream of bis(2-hydroxyethyl) terephthalate (sometimes called BHET), separating a portion of the stream, and into the separated portion adding one or more dicarboxylic acids and one or more dihydric alcohols, reacting these added compounds to form bis esters and water, removing the water, then combining the separated stream with the stream from which it was originally separated and finally polycondensing the bis compounds to form polyester.
It has been found that the just described process gives an advantage in that the "side stream" reaction allows for easy removal of water, and does not burden the downstream polycondenser with having to remove both water and glycol. Furthermore, the rate of reaction of the added dicarboxylic acid with the added dihydric alcohol is increased, especially in cases where the solubility of the acid in the dihydric alcohol is low.